The present invention relates to the art of digital communications, particularly modulation and encoding methods, specifically in which the information to be transmitted is conveyed by means of the relative position of pulses in a series of transmitted pulses or the time or spacing between pulses.
Communication, as well known, is the process of conveying information (also referred to as “data” hereinafter), from a sender, or more technically speaking a transmitter, to a receiver, via a medium. Such a medium can be a twisted pair of copper wires, a laser beam or an RF carrier, to name a few. Information can be encoded in various ways, such as an analog voltage representing human sound amplitude and frequency and pitch, or a series of digital pulses representing the geographical coordinates of a specific location, and so on. Typically, information is communicated by modulating a carrier wave, e.g. changing its amplitude or frequency or phase, in a way that uniquely encodes the transmitted information.
It is to be noted that the term “modulation” is usually related in the art to the process of varying a periodic waveform, usually a carrier wave (e.g. RF); “encoding” is usually related to transforming information from one format to another, typically at the baseband level; however in this document, those terms are alternatively used, particularly “encoding” which is used in a wider context expressing any form of transforming information from one format to another, either at the baseband level or at the carrier level or a combination thereof (such a combination is also known as modulation).
Many modulation methods are known and practiced in the art, for analog information, such as: Amplitude Modulation (AM); Frequency Modulation (FM); Phase Modulation (PM); and for digital information, such as: Amplitude Shift Keying (ASK); Frequency Shift Keying (FSK); Phase Shift Keying (PSK); and Minimal Shift Keying (MSK).
Modulation methods can be compared according to complexity of implementation and efficiency of conveying information in terms of throughput and noise immunity. Some modulation techniques enable conveying relatively much information per provided bandwidth, but might trade off the noise immunity, while others may better suit noisy channels. Accordingly, communication systems and applications adopt this or that modulation method. Over the years, less efficient modulation methods phased out and were been replaced by more efficient modulation techniques, usually digital. Typically, such digital modulation methods are implemented along with more efficient schemes of baseband data encoding, including data compression and error correction coding. Examples for this trend are military radios moving from analog FM to digital PSK or MSK or GMSK modulation achieving denser channel spacing and higher data rates per channel; cellular communication migrating from wide band analog AMP to narrow band NAMP modulation, or to digital GSM standards; Television and video cable broadcasting migrating from very wideband analog VHF and UHF channels to more compact digital modulation and data compression (e.g. MPEG) over significantly narrower broadcasting channels.
In some cases, even after applying such modern and efficient modulation schemes, a further upgrade can be achieved, to increase the system throughput. In the scope of the present invention, this typically applies to communication systems that transmit on periodic basis, at predefined transmission timing, i.e. the nominal time of transmission is basically known to the receiver, and modification of this nominal transmission timing can be interpreted as modulation of data.
An example for such a system is the satellite Search and Rescue (SAR) system known as Cospas-Sarsat. Though the present invention is not limited to this specific system, Cospas-Sarsat is a good example to clarify the present art, as well as the present invention, so it is specifically enlightened here. A detailed description of the system is provided in www.cospas-sarsat.org or www.cospas-sarsat.com.
Cospas-Sarsat is a satellite communication system to assist SAR of people in distress, all over the world and at anytime. The system was launched in 1982 by the USA, Canada, France and the Soviet Union (now Russia) and since then, it has been used for thousands of SAR events and has been instrumental in the rescue of over 20,000 lives worldwide. The goal of the system is to detect and locate signals from distress radio beacons and forward the data to ground stations, in order to support all organizations in the world with responsibility for SAR operations, whether at sea, in the air or on land. The system uses spacecraft—Low Earth Orbit (LEO) and Geostationary (GEO) satellites; and in the future also Medium Earth Orbit (MEO) satellites; Cospas-Sarsat radio beacons transmit in the 406 MHz band. The position of the beacon is determined either by the Doppler shift of the received beacon signal or by position coordinates modulated on the signal, provided by a Global Navigation Satellite System (GNSS) receiver integrated in the radio beacon.
As a skilled person probably appreciates, GNSS is usually a general term, as well as GPS (Global Positioning System) and SPS (Satellite Positioning System) and SNS (Satellite Navigation System); these acronyms may generalize particular systems such as the USA GPS or the Russian GLONASS or the European GALILEO. In the scope of the present invention, unless referring to a specific system, the terms GNSS and GPS usually relate to a generic satellite navigation system, therefore encompassing all kinds of specific navigation or positioning satellite systems.
All Cospas-Sarsat beacons are subject to the same RF specifications, yet may employ a different mechanical structure and different activation method, possibly also slight differences in the data message modulated on the signal; those differentiations usually reflect different applications, typically marine or airborne or terrestrial, so several types of beacons are defined accordingly: a) Emergency Position Indicating Radio Beacon (EPIRB) for marine use; b) Emergency Locator Transmitter (ELT) for aviation use; and c) Personal Locator Beacon (PLB) for personal and/or terrestrial use. For the purpose of the present invention, the terms EPIRB or PLB are alternatively used, and unless indicated otherwise, these terms relate to generic radio location beacons, therefore encompassing all kinds of specific location beacons.
The Cospas-Sarsat standard defines two different lengths of messages to be modulated on a 406 MHz carrier: 112 (“short message”) or 144 (“long message”) bits long.
Three levels of position resolution can be encoded in these restricted in length messages:
a) position data in the short message with a resolution of either 15 minutes (of an arc) or 2 minutes;
b) position data in the long message with a resolution of 4 minutes (“User Location Protocol”);
c) position data in the long message with a resolution of 4 seconds (“Standard Location Protocol” or “National Location Protocol”);
As a skilled person may appreciate, even the highest resolution of 4 seconds provides a worst case error of 4/60*1 NM, i.e. approximately 125 meters (1 NM=1 Nautical Mile=1853 meters). For several scenarii, e.g. searching for a person fallen overboard a vessel at a stormy dark night, even an error of 125 meters might be critical. Yet, if four more bits could be communicated to augment the Latitude and four bits to augment the Longitude, this ambiguity of 125 meters could shrink to 125/16, i.e. approximately 8 meters only. At this distance, a person may be significantly better heard and seen.
When activated, automatically or manually, a Cospas-Sarsat beacon transmits short signals, each about 0.5 seconds long, repetitively every 50 seconds plus or minus 2.5 seconds; this variation of the beacon transmission repetition rate should be random, according to the standard, to minimize transmission collisions among beacons.
It is then an object of the present invention to use this permitted variation in the transmission timing of the beacon to communicate data that augments the beacon report, particularly providing a finer resolution to the geographical coordinates reported in the beacon's message.
It is also an object of the present invention to provide a radio beacon configured to transmit data messages according to present or future protocols, using present art modulation methods, enabling conveying additional data, without modifying the present signal, but only its transmission timing.
It is further an object of the present invention to provide a receiver configured to demodulate the present art message transmitted by beacons, and also demodulate the auxiliary data communicated through variations of the transmission timing.
It is another object of the present invention to upgrade communication systems, which communicate information using any type of analog or digital modulation, such as Amplitude Modulation (AM); Frequency Modulation (FM); Phase Modulation (PM); Amplitude Shift Keying (ASK); Frequency Shift Keying (FSK); Phase Shift Keying (PSK); increasing these systems throughput by adding an auxiliary modulation that utilizes the timing of transmission of the already modulated signal.
It is yet another object of the present invention to provide a method to communicate data by modulating the predefined transmission timing of a transmitter, yet keeping a random or pseudo random variation of the transmission timing.
Many radio beacons, particularly Cospas-Sarsat beacons, comprise GNSS receivers, which in the recent years became low size and low power and low cost, thus much more popular. Integrating a GNSS receiver in the radio beacon enables locating this beacon much more accurately than using the Doppler method. Then, it is another object of the present invention to employ GNSS timing signals in the process of determining the transmission or reception timing, and possibly also use positioning data in order to calculate the difference or offset of the reception timing from the transmission timing.
Other objects and advantages of the invention will become apparent as the description proceeds.